The hemagglutination-inhibition (hereinafter briefly referred to as HI) reactions of viruses is more sensitive and more expedient to perform than neutralization and complement-fixation tests, and have been employed as a useful means of serological diagnosis. The test sera of human beings and other animals that are subjected to viral HI reactions contain nonspecific hemagglutination inhibitors and natural hemagglutinins which must be removed or inactivated before the HI reactions are carried out. The common procedure heretofore taken for this purpose is a two-step procedure which comprises adding kaolin particles to the test serum, incubating the mixture to adsorb and remove the nonspecific hemagglutination inhibitors, centrifuging the same and incubating the supernatant with erythrocytes to cause neutral agglutinins to be adsorbed on the erythrocytes. However, this two-step procedure is not sufficiently expedient in that, for example, it not only requires a plurality of incubating steps and an intermediate centrifugation step but involves the use of dissimilar buffers for carrying out those two steps.
In the face of the above technical obstacles, intensive research was conducted by the present inventor for the resolution of the above-mentioned disadvantages. The research led to the novel and unexpected finding that both nonspecific hemagglutination inhibitors and natural hemagglutinins can be removed almost completely in a very convenient procedure by pretreating the test serum with a composition containing both fixed avian erythrocytes and kaolin particles. Further research based upon the said finding resulted in the completion of the present invention.